Prosthesis for the replacement of hip joints are well known. Originally, only the ball-end on the head of the femur could be replaced but it has since proved possible to replace either part of the hip joint i.e. the acetabular socket of the joint or the ball-end on the femur.
Known acetabular cup implants, which form the socket portion of an artificial hip joint, comprise a metal cup shell, which is secured within a cavity in the pelvic bone of a patient, and an inner liner of plastic material which provides a spherical bearing surface for receiving the ball portion of the joint. The metal cup shell may be provided with an external thread to facilitate anchorage to the pelvic bone or may be secured by other means such as cement or screws.
Most current designs of metal cup shells can be grouped into two basic profiles, frusto-conical and hemispherical, since these shapes may be conveniently fabricated by rotating reamers. In all cases the designs rely upon the floor and internal walls of the acetabulum for anchorage and to transmit the forces to which the joint is subjected.
There are several designs of acetabular cup implants which comprises a continuous or discontinuous circumferential flange around the opening of the cup implant. U.S. Pat. No. 4,563,778 discloses an acetabular cup assembly having a circumferential flange. That cup assembly is adapted to be secured to bone tissue with bone cement. U.S. Pat. No. 3,982,281 discloses an acetabular cup assembly having a circumferential flange. That cup assembly is inserted into the acetabulum so that there is initially clearance between the flange and the bone structure to allow settling movement in the device before the flange contacts the bone. U.S. Pat. No. 4,180,873 discloses a frusto-conical cup shell having a circumferential flange which is force fitted into a prepared bone void of the acetabulum.
It is an anatomical fact that the thickest and strongest section of the pelvis in the acetabular region is the rim of the acetabulum. However, this rim presents an uneven and irregular margin which has precluded its use for load bearing in a hip joint prosthesis. It has now been found that by smoothing the rim of the acetabulum and providing a metal cup shell with a flange which bears on the machined surface the potential load bearing capacity of the rim of the acetabulum can be effectively utilized in addition to the internal surfaces of the acetabulum.